In recent years, electronic display devices which allow a book or the like to be displayed on a portable display device for reading, i.e., so-called electronic books, have been available in the market. Because improvements have been made in hardware functions, e.g., resolution and contrast, of electronic display media, the devices have come down in price, and the devices which do not use paper are considered to be ecological (e.g., patent document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-196012